Thursday, November 04, 2010

CMS observes ZZ production, too

In September, when the luminosity was just a few inverse picobarns per month (and not per day what it is now when the machine is running), the LHC collider managed to realize a pretty nice collision. Fortunately, the CMS detector just happened to be around to see it. Imagine, one of the relatively few ZZ productions in the Solar System ever - and a powerful detector just sits next to it. :-)

You see that four muons were created. One may reconstruct the momentum of the muons and add them - and the invariant masses show that they came from two decaying Z bosons. The CMS is good at seeing muons - muon is what the "M" in its name stands for - so it couldn't have missed the event. The event is pretty cool because the production of two Z bosons in the final state is a pretty rare process.

In fact, in the sequence of increasingly rare processes to occur at the colliders, the ZZ production is perhaps the very last, most demanding one before the Higgs:

The cross sections for increasingly rare processes decrease pretty much exponentially.

If you are as keen on the God particle as Sarah Kavassalis, a disappointment is waiting for you on the top of the diagram above. It says "Tevatron". And indeed, its D0 detector has observed three ZZ events by July 2008.

The LHC has just gotten to the same "qualitative" level in the sequence of rare electroweak processes that are similar to the Higgs boson production. However, don't expect the Higgs boson to be discovered soon. Its production is an uncommon event and the signatures are weak enough so that it will probably take a year or two.

Meanwhile, the LHC is already thousands of times more powerful a machine in the production of particles that are significantly heavier than the Z boson. SUSY and, less likely, other kinds of new physics may be less rare than the Higgs and they could have even been produced; they have surely not been publicized yet (note the more-than-month delay after the ZZ production). Its energetic advantage is responsible for this difference. The Tevatron can barely reach to the 150 GeV territory so it only rarely visits that realm.

There is some misunderstanding on the Internet (and Twitter) about the difference between ZZ production and the Higgs boson production. Well, the two Z-bosons could in principle come from a Higgs boson, too. It's just very unlikely. Why?
  1. First, the Higgs boson would have to be heavier than twice the mass of the Z-boson - above 180 GeV (the CMS folks know the right number for this event and it's probably significantly higher). Other kinds of data disfavor this "heavy" possibility. However, as we discuss later, the 180 GeV bound only works for on-shell Z-bosons. Below, we will mention off-shell Z-bosons in H-decays, too
  2. Second, the ZZ production simply works even without the Higgs and that's probably what we are seeing.
  3. Third, the Tevatron has already observed several ZZ events and they would predict different reconstructed Higgs masses if you imagine a Higgs in at least two of them. So it's unlikely.
  4. Fourth, even if there were a heavy Higgs at the beginning, it would still not rule out the existence of a light Higgs boson.
It's important to realize that from the viewpoint of the underlying theory, the ZZ production and the Higgs production are just two different processes. One of them may be more interesting or more new for us but Nature doesn't care.

Also, you shouldn't imagine that the Higgs production is always a "subset" of the ZZ production events. Quite on the contrary, the production of a new particle always subtly and statistically differs from processes involving several "old" well-known particles although they often overlap, too. Just to remind you: the Higgs particle has to be unstable and it decays in various ways. The fraction of choices is dictated largely by its mass:

Branching ratios (proportions) of Higgs decay channels (log scale) as a function of the Higgs mass.

Note that for a light Higgs below 130 GeV, the dominant decay channel is into a bottom quark-antiquark pair. That's because the other candidate final particles are heavier than 1/2 of the light Higgs mass so the decay is prohibited by energy conservation. But the Higgs wants to choose the heaviest allowed fermion because the interaction constants (Yukawa couplings) increase with the mass.

For the LHC, this key role for the bottom quark is unfortunate because it's not extremely good at detecting the bottom quarks (Tevatron is better; in some sense, the LHC is equally superior at top quarks). Also note that the other important channels are tau-antitau, charm-anticharm, Z-gamma, two gluons, and two gamma.

You might be surprised that the diagram above allows the WW and ZZ decays already approximately from 80 GeV or 90 GeV for the Higgs mass, respectively. On the other hand, the top-antitop decays are only allowed for Higgses heavier than 2x 175 GeV = 350 GeV or so.

That's because the diagram of branching ratios above also includes "off-shell" decays into particles with a wrong momentum-energy relationship - that must be virtual which means that they inevitably further decay (far enough from any possible resonance). That's also why the diagram above doesn't show any multiparticle (more than two) decay channels.

But why did your original intuition work for the top quarks whose minimum Higgs mass was indeed twice the top mass? And why we didn't have to double the mass for the W-bosons and Z-bosons? Well, it's because W-bosons and Z-bosons are bosons but the top quark is a fermion. However, it's actually not true that decays to off-shell quark-antiquark pairs are completely forbidden. They just follow a different power law because of the different spin.

To be more accurate, I would have to be more quantitative and this blog entry would look like a homework in quantum field theory which I don't want...

Via Sarah Kavassalis

No proton-proton collisions in 2010

As the US LHC blogs inform, the proton-proton collisions for 2010 ended today in the morning. It's over - and the folks are gonna go through the data.

It went so well that they're thinking about replacing 7 TeV by 8 TeV in 2011. Well, that would be unfortunate because the improvement wouldn't be too big - but it would be big enough to make the 7 TeV data incompatible with the new ones.

I propose 10 TeV, the geometric average of 7 TeV and 14 TeV. Many CERN papers have actually calculated with 10 TeV as the intermediate step.

1 comment:

  1. On your first reason why it is unlikely to come from a Higgs surely you mean: the Higgs boson (rather than Z) would have to be heavier than twice the mass of the Z-boson